Exit wounds appear as a much larger cavity to entry wounds. As a bullet hits a human body, it is exerting pressure upon a small point that is support by the mass of the rest of the body in front and to the sides of where the bullet enters. This mass exerts a force in the reverse direction to the bullet's trajectory, supporting surrounding tissue and thus keeps the wound small. The bullet pushes flesh to the sides of its path.
When a bullet exits a human body, as it nears the surface the mass around where the bullet will exit is completely unsupported, as it exits there is nothing exerting a force in the reverse direction from the bullet, and because of the lack of support for surrounding mass in the opposite direction, a large amount of flesh is pushed away and loose from the body, causing a larger wound.
This is also coupled with the way a bullet's shape and orientation can change as it passes through a body. A 5.56 round is long and thin, and would typically enter lengthways into the body, but before it leaves the body it is possible for the bullets orientation to be changed (through hitting bones) so that it's exit is widthways, exerting force over a larger surface area.